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Range-Bounding Normal Fault of Smith Valley, Nevada: Limits on Age of Last Surface-Rupture Earthquake and Late Pleistocene Rate of Displacement

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Range-Bounding Normal Fault of Smith Valley, Nevada: Limits on Age of Last Surface-Rupture Earthquake and Late Pleistocene Rate of Displacement *Manuscript Click here to download Manuscript: 2010-12-12-SmithValleyMS-FINAL.doc Range-Bounding Normal Fault of Smith Valley, Nevada: Limits on Age of Last Surface-Rupture Earthquake and Late Pleistocene Rate of Displacement. Steven G. Wesnousky Center for Neotectonic Studies University of Nevada, Reno 89557 Marc Caffee, Dept. of Physics, PRIME Lab Purdue University Lafayette, IN 47906 electronic supplement: Two tables with details of inputs for radiocarbon and cosmogenic analyses and photos of rocks sampled for cosmogenic analyses. Abstract Smith valley is bounded on it’s western edge by an active normal fault and the Pine Nut mountains. Displacement on the fault is responsible for development of one of the westernmost basins of the Basin and Range province of the western United States. Interpretation of an exposure afforded by excavation of a trench across the range- bounding fault places the most recent surface rupture earthquake after 5176 ± 130 cal BP and suggests it was close in time to 3530 ± 82 cal BP. Utilization of tephrochonologic and cosmogenic analyses and mapping of fault scarps across the Artesia Road fan are the basis to put forth an initial estimate of the late Pleistocene vertical slip rate of the fault at between 0.125 mm/yr and 0.33 mm/yr. Introduction Smith Valley is one of the westernmost basins of the Basin and Range province and is located in the Walker Lane, a zone of disrupted topography that is generally characterized by north-northwestward oriented ranges and faults that follows the 1 eastern edge of the Sierra Nevada (Figure 1). Geodesy shows that ~1 cm/yr of Pacific- North America plate motion is localized in the Walker Lane (e.g., Thatcher et al., 1999). With the intent of contributing information to seismic hazard analysis in the region and ongoing efforts to understand where and what portion of accumulating geodetic strain is released in earthquakes (e.g. Hammond and Thatcher, 2007) , this note puts forth observations bearing on the age of the last surface rupture earthquake and late Pleistocene slip rate of the Smith Valley fault. Smith Valley is bounded on it’s western edge by an active normal fault and the Pine Nut Mountains (Figure 2). The valley floor sits at about 1400 m and the Pine Nut Mountains reach elevations of ~2700 m. The fault is locally expressed by an abrupt rangefront, triangular facets, and scarps in young fanhead alluvium from a southern limit of ~38° 40’ northward a distance of ~30 km to ~ 38° 55’ (Figure 2). At the latitude of ~ 38° 55’ the fault may step eastward into the valley and scarps in young alluvium appear absent along a more sinuous rangefront. Scarps in young alluvium are again evident further north in the narrow valley between the Pine Nuts and the Buckskin Range, extending to ~ 39° 06’, bringing the total length of the fault system to 45-50 km. Our observations result from the study of two sites along the fault zone. Site 01 is located a couple of km north of where the West Walker river enters Smith Valley. Site 02 is located about 14 km to the north (Figure 2). 2 Upper Colony Road Trench - Site 1 The fault trace follows a small re-entrant just north of where the West Walker River enters Smith Valley (Figure 2). Here the fault is manifested by a clear ~3.5 m scarp in fan alluvium (Figure 3). A sketch of the fault exposure produced by cutting a trench across the scarp is shown in Figure 4. The lowest beds across the exposure are designated Unit 1 and composed of alternating subhorizontal and commonly discontinuous beds of fine and coarse granitic gravel. Unit 1 is interrupted and displaced by several faults in the central portion of the log. The offset of Unit 1 across the westernmost trace has dropped Unit 1 downward east of the fault and produced a small triangular graben. The materials within and above the graben are labeled Unit 2 and consist of granitic gravel identical to Unit 1 but distinguished by a lack of bedding and massive character. Localized and weak cut and fill fabric within Unit 2 is interpreted to suggest emplacement was by alluvial as well as colluvial processes. Unit 2 deposits are interpreted to have accumulated subsequent to the fault displacement. The radiocarbon age of charcoal sampled from a burn horizon near the base of unfaulted Unit 2 is 3530 ± 82 cal BP (sample UC7 in Figure 4 and Table S1 available as an electronic supplement to this paper). A charcoal sample taken from the faulted Unit 1 yielded a radiocarbon age of 5176 ± 130 cal BP (sample UC2 in Figure 4 and Table S1 available as an electronic supplement to this paper). The observations indicate that the most recent surface displacement occurred after 5176 ± 130 cal yr BP. The location of UC7 at the base of unit 2 is reason to suggest the last displacement was close in time to 3530 ± 82 cal BP. The similarity of the offset (thickness of colluvial wedge) observed in the trench to the scarp height observed on the surface (3.5 m) 3 suggests that the scarp is primarily if not completely due to a single earthquake displacement. Artesia Road Fan - Site 02 To the north of the West Walker River the fault takes a sharp right bend. Here an unnamed drainage is the source of the moderate sized Artesia Road fan on which scarps produced by the range-bounding fault are clearly expressed (Figures 2 and 5). In Figure 6, the surficial geology, scarp locations, vertical separation profiles, and annotated sample localities are placed on a 2-m contour base map. The fan is composed of granitic gravel. Surface characteristics of color and morphology allow division of the fan surface into 3 units of progressively greater age. The youngest of these (Qy) is a small fan cone composed of angular cobble and boulder gravel and the modern drainage channel from which it emanates. The drainage channel and fan cone of Qy respectively cut and rest upon the intermediate age unit Qi. The unit Qi surface is characterized by a bright white tone resulting from generally unweathered and poorly sorted angular and subangular boulders in a poorly sorted sand to cobble matrix. Debris flow levees and channels are interspersed within the bar and swale topography of the Qi surface. Qo, the oldest surface, is distinguished from Qi by its red tone, smooth surface, and a lesser frequency, smaller size, and common absence of cobbles and boulders. Measurements of the cumulative vertical separation across the scarps range from ~ 7 to 10m and 16 to 22 m across the younger Qi and older Qo surfaces, respectively (Figure 6). For simplicity of discussion the variability in the estimates of vertical separations measured across the scarps are ignored and offsets of the Qi and Qo surfaces are taken herein to be 10m and 20 m, respectively. 4 Age estimates of the fan surface and thus and thus limits on the rate of fault displacement result from 1) tephrochronology of lacustrine beds on which the fan has developed, and 2) the calculated cosmogenic surface exposure ages of several boulders on the fan surface. Tephrochronology The sample SGW-SV1-2008 was sampled along a roadcut exposing fine- grained gray lacustrine sediments below the alluvial Qo surface (Figure 6). Electron Microprobe (EM), chemical, and statistical analyses was performed by the United States Geological Survey Tephrochronology Project in Menlo Park, California. EMA methodogy is described in Sarna-Wojcicki et al. (1984). Data evaluation methods are based on the similarity coefficient of Borchardt (1974), and Borchardt et al. (1972). For a more complete and detailed description of tephrochronologic methods, see Sarna-Wojcicki (2000), and Sarna-Wojcicki and Davis (1991). Analytical results show that SGW-SV1- 2008 closely matches (~0.96 similarity coefficient) a Pleistocene tephra (~75-80 ka, correlated age range) recovered from from Owens Lake in southeast California . The correlated age-range is based on a sedimentation rate curve presented by Bischoff et al (1997) (Elmira Wan, U.S. Geological Survey, pers. comm.). Stauffer (2003) reports the analysis of a tephra (sample i.d. 99AL617-154) from lacustrine sediments across the valley to the east and interpreted to have been deposited by the same lake. Analyses also show that 99AL617-154 matches well (0.95 to 0.97 similarity coefficient) with several samples from cores in Walker Lake. The Walker Lake core samples have estimated ages of 80-60 thousand years years ago (during MOI Stage 4) based on sedimentation-rate estimates for Walker Lake (Stauffer, 2003 reporting 2003 personal communication of Sarna-Wojcicki, U. S. Geological 5 Survey). AL98-51A is another MOIS 4 (70-60 ka) tephra layer within the same lacustrine beds at the south end of the map area of Figure 6 (collector: Stauffer (2003)). Unfortunately, the geochemical fingerprint of AL98-51A does not compare as well as the other samples. Nevertheless, the synthesized tephrochronology places the development of the fan subsequent to about 80,000 to 60,000 years ago. Cosmogenic Analysis Several granitic boulders were sampled from the older Qo fan surface with the aim of determining the length of time each has been exposed to cosmogenic radiation at the earth surface. Estimating the age entails measurement of the quantity of the isotope 10Be accumulated in the quartz fraction of the rock. A review of the theory, method, and various applications of the method are found in Gosse and Phillips (2001). The geographic and production rate parameters, further references to the specifics of the methodology used for calculating the exposure ages of the boulders, and field photos of the particular samples are provided in Table S2 available as an electronic supplement to this paper.
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